PT-2026-64669 · Linux · Linux

CVE-2026-64448 · Published 2026-07-25 · Updated 2026-07-25

Mitre

In the Linux kernel, the following vulnerability has been resolved:

smb: client: restrict implied bcc[0] exemption to responses without data area

smb2 check message() has a long-standing quirk that accepts a response whose calculated length is one byte larger than the bytes actually received ("server can return one byte more due to implied bcc[0]"). This was introduced to accommodate servers that omit the trailing bcc[0] overlap byte when no data area is present.

However, the exemption is applied unconditionally, regardless of whether the command actually carries a data area (has smb2 data area[]). When a response with a data area is subject to the +1 exemption, the reported data can extend one byte beyond the bytes actually received, yet smb2 check message() still accepts it. The subsequent decoder then reads past the end of the receive buffer. This is reachable during NEGOTIATE and SESSION SETUP, before the session is established.

The resulting out-of-bounds reads are visible under KASAN when mounting against a non-conforming server; both the SPNEGO/negTokenInit and the NTLMSSP challenge decoders are affected:

BUG: KASAN: slab-out-of-bounds in asn1 ber decoder+0x16a7/0x1b00 Read of size 1 at addr ffff8880084d67c0 by task mount.cifs/81 CPU: 1 UID: 0 PID: 81 Comm: mount.cifs Not tainted 7.1.0-rc6 #1 Call Trace: dump stack lvl+0x4e/0x70 print report+0x157/0x4c9 kasan report+0xce/0x100 asn1 ber decoder+0x16a7/0x1b00 decode negTokenInit+0x19/0x30 SMB2 negotiate+0x31d9/0x4c90 cifs negotiate protocol+0x1f2/0x3f0 cifs get smb ses+0x93f/0x17e0 cifs mount get session+0x7f/0x3a0 cifs mount+0xb4/0xcf0 cifs smb3 do mount+0x23a/0x1500 smb3 get tree+0x3b0/0x630 vfs get tree+0x82/0x2d0 fc mount+0x10/0x1b0 path mount+0x50d/0x1de0 x64 sys mount+0x20b/0x270 do syscall 64+0xee/0x590 entry SYSCALL 64 after hwframe+0x77/0x7f Allocated by task 85: kmem cache alloc noprof+0x106/0x380 mempool alloc noprof+0x116/0x1e0 cifs small buf get+0x31/0x80 allocate buffers+0x10d/0x2b0 cifs demultiplex thread+0x1d5/0x1d50 kthread+0x2c6/0x390 ret from fork+0x36e/0x5a0 ret from fork asm+0x1a/0x30 The buggy address is located 0 bytes to the right of allocated 448-byte region [ffff8880084d6600, ffff8880084d67c0) which belongs to the cache cifs small rq of size 448

BUG: KASAN: slab-out-of-bounds in kmemdup noprof+0x36/0x50 Read of size 329 at addr ffff88800726c678 by task mount.cifs/89 CPU: 0 UID: 0 PID: 89 Comm: mount.cifs Tainted: G B 7.1.0-rc6 #1 Call Trace: dump stack lvl+0x4e/0x70 print report+0x157/0x4c9 kasan report+0xce/0x100 kasan check range+0x10f/0x1e0 asan memcpy+0x23/0x60 kmemdup noprof+0x36/0x50 decode ntlmssp challenge+0x457/0x680 SMB2 sess auth rawntlmssp negotiate+0x6f0/0xcb0 SMB2 sess setup+0x219/0x4f0 cifs setup session+0x248/0xaf0 cifs get smb ses+0xf79/0x17e0 cifs mount get session+0x7f/0x3a0 cifs mount+0xb4/0xcf0 cifs smb3 do mount+0x23a/0x1500 smb3 get tree+0x3b0/0x630 vfs get tree+0x82/0x2d0 fc mount+0x10/0x1b0 path mount+0x50d/0x1de0 x64 sys mount+0x20b/0x270 do syscall 64+0xee/0x590 entry SYSCALL 64 after hwframe+0x77/0x7f Allocated by task 93: kmem cache alloc noprof+0x106/0x380 mempool alloc noprof+0x116/0x1e0 cifs small buf get+0x31/0x80 allocate buffers+0x10d/0x2b0 cifs demultiplex thread+0x1d5/0x1d50 kthread+0x2c6/0x390 ret from fork+0x36e/0x5a0 ret from fork asm+0x1a/0x30 The buggy address is located 120 bytes inside of allocated 448-byte region [ffff88800726c600, ffff88800726c7c0) which belongs to the cache cifs small rq of size 448

Restrict the +1 exemption to responses that have no data area, so that it still covers the bcc[0] omission it was meant for. When a data area is present, the +1 discrepancy instead means the reported data length overruns the ---truncated---

Related Identifiers

CVE-2026-64448

Affected Products

Linux

References · 9

• https://nvd.nist.gov/vuln/detail/CVE-2026-64448 · Security Note
• https://git.kernel.org/stable/c/8d0bbc78046d264bbf6a574ea6f9072258a43e35 · Note
• https://git.kernel.org/stable/c/419ec1b604d7fb60c10aec2dc062371f9fcd4940 · Note
• https://git.kernel.org/stable/c/53b7c271f06be4dd5cfc8c6ef552a8355c891a7f · Note
• https://git.kernel.org/stable/c/b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0 · Note
• https://git.kernel.org/stable/c/ceb875a375dedbf51c9425c1d13a2d7a8435c08c · Note
• https://git.kernel.org/stable/c/6e9d10f62773b99bd927940fd9cbdfe7207e23ff · Note
• https://git.kernel.org/stable/c/573e502d14714d2947e22e7eff40ec20a6a44a42 · Note
• https://git.kernel.org/stable/c/31c6312608c60b72a1feb99a5afb680645a3e8a3 · Note